In a cellular communication system, a subscriber unit communicates within a "cell" or "beam" which contains multiple channels. When a subscriber unit exits a first cell, the call is handed off to another cell which is projected by the same or a different cellular antenna. Channel frequencies within adjacent cells are typically selected by the communication system so that they do not interfere with each other.
Prior art terrestrial cellular communication systems are described in William C. Y. Lee, Mobile Cellular Telecommunications (2d ed. 1995). Prior art terrestrial cellular communication systems provide communication channels to subscriber units by projecting fixed cellular beams from cellular antennas located on 30-100 foot towers. A cell's size depends on power, antenna type, geometry, and local geography. The more power emitted by the antenna, the greater the size of the cell. However, increased power produces more interference. An omnidirectional, omnicell antenna projects an ideally circular cell around the cellular antenna. A directional antenna allows a cell pattern to extend in a planned direction relative to the antenna.
Terrestrial cellular systems have several disadvantages. For example, only limited service areas are feasible because of the necessity of antenna towers. With antenna ranges of twenty miles or less, coverage of rural and remote areas would not be cost-effective. Also, once an omnidirectional or directional antenna is set up, only a hardware change can affect the shape or location of the cell or the frequencies assigned to the communication channels.
Satellite cellular communication systems have been proposed to overcome some disadvantages of terrestrial cellular systems. For example, a satellite cellular communication system using low earth orbit (LEO) satellites can provide a much greater service area than a terrestrial system because the satellites can project cells toward the entire surface of the earth. Several proposed satellite cellular systems project "satellite-fixed" cells toward the surface of the earth. FIG. 1 illustrates the motion of a satellite-fixed cellular footprint relative to a hypothetical urban area 18 as a prior-art satellite 10 progresses in its orbit. At time=1, the satellite's cellular footprint is projected toward area 12. At time=2, after satellite 10 has traveled along orbital path 16, the satellite's cellular footprint is projected toward area 14. Relative to satellite 10, the direction that the cellular footprint is projected does not change. However, relative to the earth, the cellular footprint travels smoothly over the earth's surface as satellite 10 progresses in its orbit.
During a typical conversation, several cells and/or satellites might pass over a particular subscriber unit. Because of the relative movement of the cells to the subscriber units, frequent cell-to-cell handoffs are required. In addition, when a satellite moves out of range of a particular satellite, a satellite-to-satellite handoff is required. These frequent handoffs add a level of complication to the management of the system.
Earth-fixed satellite cellular beams have also been proposed. In a system employing earth-fixed satellite cellular beams, the satellites steer their antennas to project a footprint toward a particular region of the earth. These regions are predetermined by the system and typically represent high-demand areas (e.g., major metropolitan areas or continental regions). FIG. 2 illustrates an earth-fixed cellular footprint relative to a hypothetical urban area 28 as a prior-art satellite 20 progresses in its orbit. At time=1, time=2, and time=3, satellite 10 projects its cellular footprint toward area 22. As satellite 20 moves along its orbital path 24, the cellular footprint is "steered" either electrically or mechanically toward area 22. The shape of area 22 might vary as satellite 20 moves, but the direction of the cellular footprint projection remains relatively constant. The shape of area 22 is nearly circular when satellite 20 is at a high angle of elevation (e.g., at time=2). When satellite 20 projects the satellite footprint from a low angle of elevation (e.g., at time=1 and time=3), however, the shape of the satellite footprint and the cells contained within the footprint are elliptical. Thus, during the pass of a satellite, the shape of a particular cell changes from an ellipse to a circle and back to an ellipse.
Examples of prior art, earth-fixed satellite cellular systems are described in U.S. Pat. Nos. 5,415,368 and 5,439,190, where Horstein, et al. disclose a "coordinated boresight steering" method where the communication system determines antenna focal directions for projection of a satellite footprint for a predetermined orbital period. The focal directions are adjusted during the orbital period so that the footprint is steered toward a particular geographical region during the orbital period. Another example of an earth-fixed satellite cellular system is described in U.S. Pat. No. 5,408,237, Patterson, et al.
Earth-fixed systems can minimize cell-to-cell handoffs and can simplify satellite-to-satellite handoffs. However, a drawback to earth-fixed satellite cellular systems is that these systems do not compensate for demand variations. The action in which a footprint is projected is predetermined. Thus, these systems cannot react in real time to varying subscriber demands. Although an earth-fixed cellular system can target a high demand area, each cell has a fixed capacity. Thus, only a number of subscribers not exceeding that cell's capacity can use the system at one time.
Another negative aspect to earth-fixed systems is that they continuously project RF energy toward the earth, whether or not communication channels are being used. This results in unnecessary RF interference over a large area.
A drawback to both terrestrial and satellite prior art systems is that they waste power by projecting a fixed footprint of cells toward a region, without taking user demand into account While some cells can see very high demand, other cells can see little or no demand. Thus, wasted power is expended in projecting cells toward areas with little or no demand. Power considerations are especially important in satellite communication systems because of limited battery storage capabilities of the satellites.
What is needed is a method and apparatus to increase the traffic-carrying capacity of a cellular communication system while minimizing power consumption and unnecessary RF interference. Further needed is a method and apparatus to better service geographically varying subscriber demands and to simplify handoff problems.